You can use a lower from wherever. Aero Precision makes more than 50% of other manufacturers lowers today... they are a good option and can usually be had less than $100.

I would suggest buying a prebuilt upper and building out your lower as you see fit. It's easy and requires very little tools. The upper requires specialized tools so you won't save any money there - at least on yoru first build. You won't beat a Daniel Defense cold hammer forged barrel... so if you don't want to get a standard DD upper... you can get one of the uppers from AIM Surplus that include DD barrels. Definitely a good option.